Continuous wave,fluorescent solid lasers

ABSTRACT

A long lasting continuous-wave, solid laser of relatively low pumping power, on the order of 1 kilowatt, comprising a rod of laser active material having at least one absorption band in the near infrared range from 0.75 to 0.97 microns and an elongated arc quartz lamp filled up with krypton under a pressure between 1.5 and 6 atmospheres and having a length 1 and a diameter d whose product 1d is larger than W/300 pi , W being the input power of the lamp. The rod and the lamp are disposed along the focal lines of an elliptical cylinder reflector.

Inventor Otto Deutschbeln 8 rue Gueudin, Montrouge, France Appl. No.9,479 Filed Feb. 9, 1970 Patented Aug. 31, 1971 Priority July 13, 1966France 69,421 Continuation-impart of application Ser. No. 652,050, July10, 1967.

CONTINUOUS WAVE, FLUORESCENT SOLID LASERS References Cited UNITED STATESPATENTS 2,965,790 12/1960 lttig et a1 313/217 3,258,716 6/1966 Nassau etal.... 33l/94.5 3,337,762 8/1967 Vincent 313/7 3,353,115 11/1967 Maiman331/945 Primary ExaminerRonald L. Wibert Assistant ExaminerT. MajorAnorney-Abraham A. Saffitz ABSTRACT: A long lasting continuous-wave,solid laser of relatively low pumping power, on the order of l kilowatt,comprising a rod of laser active material having at least one absorptionband in the near infrared range from 0.75 to 0.97 microns and anelongated arc quartz lamp filled up with krypl Chim 2 Drawing Figs tonunder a pressure between 1.5 and 6 atmospheres and hav- U.S. Cl 331/945ing a length l and a diameter d whose product 1d is larger than lnt.ClH015 3/09 W/30O1r, W being the input power of the lamp. The rod andField of Search 331/945; the lamp are disposed along the focal lines ofan elliptical 313/184 cylinder reflector.

' z-zzcriopzs \\7P4 WATER [AS/FR STPEAM CONTINUOUS WAVE, FLUORESCENTSOLID LASERS This is a continuation-in-part of copending US. Pat.application Ser. No. 652,050, filed July 10. l967.

The present invention relates to an improvement to the continuous wave,fluorescent solid laser comprising essentially, in known manner, a rodof laser material, which will produce stimulated emission of radiation,an excitation lamp which pumps power into the laser material and anoptical system for focusing the optical exciting flux emitted by saidlamp, on to the rod.

Different types of excitation lamps and associated reflectors have beenproposed in the prior art for pumping lasers to obtain high energy leveloutput in very short times. Particularly helical lamps and coaxial lampswith cold electrodes encompassing the laser rod with circularcylindrical reflectors surrounding the lamp have already been disclosedfor flash operation. The energy consumption of these lamps per eachflash is of the order of 280 joules in the helical lamp for flashes of0.1 ms. and of the order of 150 joules in the coaxial lamps for flashesof 4.5 microseconds in the case of a ruby laser containing paramagneticchromium ions. This large feed energy is due in part to the circularcylindrical reflector which does not focus the whole optical flux of thelamp onto the laser rod and also in the particular case of the helicallamp to the fact that the optical energy is radiated at discrete pointsseparated by the helix pitch along the laser rod length. Due to theirlarge energy consumption and their high current density these types ofexcitation lamps are not convenient for laser continuous wave operationand existing continuous wave lasers do not actually use them.

It has also been proposed to excite an elongated laser active member byan elongated lamp, the member and the lamp being located along the focallines of an elliptical cylinder. In this structure all the rays radiatedby the lamp are focused onto the laser member whatever reflections therays may encounter. Due to the optical energy transfer withoutdissipation loss from the excitation lamp to the active medium, thepulse energy consumption of the lamp in flash operation is lesser thanin the case of coaxial and helical excitation lamps, say 16 joulesinstead of 150 joules for the coaxial lamps which is times lesser. Itresults therefrom that elongated excitation lamps in ellipticalreflectors are well adapted to produce continuous coherent light beamsby laser action.

ln the present state of the art, a type of laser which best satisfiesthe optimum conditions to be achieved is described in the article Theoperational characteristics of a cw Nd; Ca

W0 laser in the range of dry ice to room temperature" by H. R. Aldag, R.S. Horwath, C. B. Zarowin, in Applied Optics}? Optical Society ofAmerica, New York, vol. 4, No. 5, May 1965, pages 559 to 563.

The laser described in this article comprises:

a laser rod of calcium tungstate (Ca W0 doped with neodymium, with alength of 35 mm. and a diameter of 3 mm.;

an elongated mercury exciting lamp of the GEA-H6 or A 1679 PEK type;

an optical system for focusing the optical exciting flux consisting of aright elliptical cylinder, the laser rod and the pumping tube beingdisposed respectively along the focal lines of said cylinder.

The rod and the tube are cooled by a suitable liquid coolant,

for example water.

An examination of FIG. 6 of the article cited shows that at ambienttemperature (20 C.

the laser threshhold" of the unit in question is reached for a power of1250 watts supplied to the mercury tube;

beyond there, a power of 1450 watts supplied to the mercury tube istranslated into emitted laser power of 0.050 watt.

This laser therefore has a total energy yield of Other luminous sourcesknown which are suitable for use for the stimulation of a continuouswave laser are:

the tungsten filament incandescent tube in an iodine atmosphere (DWYtube), notably through the article entitled Laser oscillations inNd-doped yttrium aluminum, yttrium gallium and gadolinium garnets," byJ. E. Geusic, H. M. Marcos, L. G. Van Uitert, in Applied physicsletters," American Institute of Physics, New York, vol. 4, No. 10, 15thMay 1964, pages 182 to 184;

the xenon arc lamp, notably through the article entitled A roomtemperature continuous Ca WO.,-Nd laser by A. A. Kaminskit, L. S.Kornienko, G. V. Maksimova, V. Vosiko, A. M. Prokhonov, G. P. Shipulo,in Soviet Physics J.E.T.P." American Institute of Physics, New York,vol. 22, No. 1, Jan. 1966, pages 22 to 25. The percent of this power,i.e. a power of 2.6Xl.4==3.6 kilowatts, gives rise to an increase ofoutput power of several tens of milliwatts. This results in an energyefficiency of about 10 It is well known that the optical incident energyflux in watts/cm. at the laser crystal surface must exceed apredetermined threshold value and that, since the optical pumping energyis not entirely converted into laser emission energy, the residualfraction of this optical energy which appears in the form of heat posestechnological problems regarding its removal which are difficult tosolve, which leads to limitation as regards the pumping power per volumeunit of the excitation lamp and consequently, as regards the poweremitted per unit of volume of the lasermaterial.

The actual pumping yield depends essentially on three factors, namely:

the power yield of the excitation lamp, that is to say the ratio betweenthe total radiant output of the lamp and the input electrical powerconsumed to feed it;

the efficiency of the optical focusing system adapted to focus themaximum of the total optical flux radiated by the excitation lamp to thelaser material;

the "spectral adaptation, since the frequencies of the emission lines ofthe excitation lamp should correspond as well as possible to thefrequencies of the absorption bands of the laser material used.

The general object of the invention is to provide a laser capable ofcontinuous wave operation in the infrared range and requiring arelatively low pumping power, of the order of l kilowatt or slightlymore.

Another object of the invention is to improve the ratio of the emissionpower of a continuous wave laser to the supply power of the excitationlamp of the laser and to lower the current density in the lamp.

A more particular object of the invention is to provide an excitationlamp which has a sufficient spectral efficiency at wavelengthscorresponding to the absorption spectrum of the laser material althoughhaving outside said absorption spectrum conversion efficiencies lowerthan excitation lamps of the prior art.

Another object of the invention is to increase the lifetime of laserexcitation lamps.

The laser of the invention comprises a laser material in the form of arod having at least one absorption band in the near infrared range from0.75 to 0.97 a, an elongated long arc krypton filled lamp, and areflector in the form of an elliptical cylinder, the laser rod and thelamp being located along the focal lines of the reflector. Preferablythe rod and the lamp have the same diameter and the interelectrodedistarice is equal to the rod length.

Krypton filled lamps are not in production at the present time becausethey have a low radiant efficiency in the visible spectrum region.Nevertheless, it is relatively easy to obtain it on specificationssimilar to those for the high-pressure xenon long-arc lamps, for exampleof the OSRAM XBF-1000 W- type, simply by replacing this latter gas withkrypton and accepting the slight modifications in the electricalcharacteristics involved in this replacement.

Xenon lamps for illumination are characterized by a powerful luminousemission with a relatively low output of heat, the color temperature ofthe emitted light being very close to that I of daylight and independentof variations in the supply voltage.

These characteristics remain if the xenon is replaced by krypton exceptfor the fundamental difference that the krypton filled lamp has apowerful emission in the near infrared region, that is to say in theregion of radiations having wavelengths comprised between 0.75 and 0.97p. (see FIG. 1 Surprisingly, krypton has also quite a poor spectralpower outside said range. For example, the conversion efficiency is 5.6percent in the 0.75O.97 1.1. range and always lower than 1.9 percent inthe ranges 0.35-0.75 p. and 0.97-1.05 u. Krypton lamps are thereforeextremely valuable in the continuous wave laser art if the lasermaterial has strong absorption bands in this same region.

The invention will be better understood from the following descriptionwhich refers to the accompanying drawings wherein:

FIG. 1 is a comparative diagram of the intensities (in logarithmicscale) of the most intense emission lines of various excitation lamps,of the mercury, xenon and krypton types respectively (diagram alreadyused in the above explanation);

FIG. 2 is a diagrammatic side view of a laser unit according to theinvention.

Referring now to FIG. 2, a laser unit was tested which comprises a laserrod 1 of calcium tungstate doped with neodymium, with a length of 50 mm.and a diameter of 3 mm. and a high-pressure long-arc krypton filledexcitation lamp 2, that is to say with a filling pressure comprisedbetween 1.5 and 6 atmospheres. The envelope of the lamp is in quartz andhas an internal diameter of 3 mm., with a wall thickness of 1 mm. and anarc length of 50 mm. The lamp is fed by the alternating voltage of 220v. of the mains through a conventional self-induction coil and it iswater-cooled.

The rod 1 and the lamp 2 are cooled with water which circulates asindicated by the arrows in FIG. 2.

The optical focusing system consists essentially of a right ellipticalcylinder 3, the rod and the lamp being disposed respectively along thefocal lines of this cylinder, and of end plates 4 or plane frontalmirrors.

This unit afforded the following results at ambient temperature (+20C.):

The R.M.S. voltage between the electrodes is 90 volts; the current inthe lamp is l l a. and the temperature of the electrodes in operation isabout 1000 C.

The laser threshold is reached with a power of 300 watts supplied to thekrypton lamp. Beyond that, a power of 1000 watts supplied to the kryptonlamp is translated into a laser power of 0.9 watt which corresponds toan overall energy efficiency of 0.9/l000=9 l0 to be compared to 3.45XIin the case of the laser of H. R. Aldag et al. and to 10' in the case ofthe laser of Kaminskit et a1. Further, the threshold power is lower thanthat of Aldag et al.s laser and lower than the laser of Kaminskit et al.The power density per volume unit of the lamp is The normal input powerrange is from 1 to 3 kw.

On the other hand the thennal load per surface unit of the wall of thedischarge lamp is With such a thermal load, there is no risk of thequartz being destroyed by melting or devitrification.

With a supply power of 1 kw., the krypton lamp has a lifetime exceeding1,000 hours; after 1,000 hours the radiant power is lowered by 20percent.

The voltage of the krypton lamp IS limited by the risk of electrodesputtering which occurs if the energy of the impacting charge carriersis too high. In the krypton lamps of the invention, the upper limit ofthis voltage is volts.

Increasing of the krypton pressure leads to an increase of the laserpower output for a given lamp power input and also to an increase of thedynamic resistance of the lamp. Higher voltages are necessary to obtainthe same power input. Limitation of the lamp voltage to 150 voltsinvolves a correlative limitation of the krypton pressure to 6atmospheres.

The thermal load of the lamp wall remain lower than the load causingdamaging of the lamp, e.g. the load at the lamp wall must be less than300 w./cm. for quartz of conventional thickness (=1 mm.). This meansthat the power density per cm. which is equal to W divided by 1r d1,where W is the power input wattage of the lamp, d is its diameter and lis its length, is equal to or lower than 300 watts per cm. In the caseabove described, (i=3 mm. and 1=50 mm. and the above condition =220 w.per cm.

$300 watts per cm.

involves W l .4 kilowatt.

The laser material used to form rod 1 can be calcium tungstate dopedwith trivalent neodymium, yttrium aluminum garnet doped with neodymiumor a calcium or strontium fluoride crystal doped with bivalentdysprosium.

I claim:

1. A long-lasting, continuous-wave, solid laser of relatively lowpumping power, on the order of l kilowatt, comprising:

a. a rod of laser material selected from the group consisting of calciumtungstate doped with neodymium, yttrium aluminum garnet doped withneodymium, calcium fluoride doped with dysprosium, and strontiumfluoride doped with dysprosium, said las rod having at least oneabsorption band in the near infrared range from 0.75 to 0.97 microns;

b. lamp and reflector means for focusing onto said laser rod excitingradiations for optically pumping said laser rod;

. the lamp of said lamp and reflector means consisting of an elongatedgenerally cylindrical excitation discharge lamp formed of a quartzenvelope filled with krypton which is under pressure of about 1.5 to 6atmospheres, said lamp being supplied with an input voltage up to about150 volts and a power input wattage, W, between about 1 and 3 kilowatts,the diameter d and length l of said lamp providing a product dl equal toor more than said total input wattage, W, divided by 300w;

d. the reflector of said lamp and reflector means consisting essentiallyof a right elliptical cylinder and an end plane mirror; and,

e. said laser rod and lamp being disposed respectively along the focallines of said right elliptical cylinder between said cylinder and saidend plane mirror, the diameter of the laser rod being the same as thediameter of the lamp and the interelectrode distance being equal to thelength of the laser rod.

